1. Field of the Invention
This invention relates to a light emitting element. In particular, this invention relates to a light emitting element including a semiconductor substrate opaque to a light emitted from an active layer and a reflecting part disposed between the semiconductor substrate and the active layer so as to have a thickness of not less than a predetermined value.
2. Description of the Related Art
Conventionally, a light emitting element is known that includes an n-type GaAs substrate, a light reflection layer disposed on the n-type GaAs substrate, an n-type Al0.45Ga0.55As cladding layer disposed on the light reflection layer, a p-type GaAs active layer disposed on the n-type Al0.45Ga0.55As cladding layer, a p-type Al0.45Ga0.55As cladding layer disposed on the p-type GaAs active layer, and a p-type GaAs cap layer disposed on the p-type Al0.45Ga0.55As cladding layer, wherein the light reflection layer has a laminated structure of an n-type AlAs/an n-type AlxGa1-xAs, the laminated structure is formed so as to have a chirp-like shape where the film thicknesses are sequentially changed, and a relationship among a changed thickness ratio, a number of the laminated layers, and a mixed crystal ratio is defined, by which a predetermined reflection wavelength width and reflectance can be obtained. The technique is disclosed in, for example, JP-A-5-37017(JP-A1993-37017).
The light emitting element disclosed in the JP-A-5-37017 has the light reflection layer capable of reflecting a light traveling to a side of the substrate by light wave interference, so that light emission output can be enhanced.
However, although the light emitting element disclosed in the JP-A-5-37017 is capable of broadening reflection wavelength region in a single body of the light reflection layer, if the light reflection layer has a thickness of not less than a predetermined value, the light emission output can not be increased, similarly to a case of using a light reflection layer that does not have the chirp-like shape. Consequently, the light emitting element of the JP-A-5-37017 discloses that the changed thickness ratio DD is preferably almost 0.1 to 0.15, thicknesses of the pair layer are preferably T (1−DD) and T (1+DD), if standard thickness is defines as T, and the number of the laminated layers of the light reflection layer is preferably almost 20 to 30. Here, if a light emission wavelength is set to 631 nm in the composition of the light emitting element disclosed in the JP-A-5-37017, a thickness of the light reflection layer becomes almost 2 to 3 μm. Namely, if the thickness of the light reflection layer becomes not less than 2 μm, enhancement of the light emission output is saturated in the light emitting element disclosed in the JP-A-5-37017, and there is a limitation to the enhancement of the light emission output.